1. Field of the invention
This invention relates to a reactive power compensator for an electric power system having a voltage control circuit which amplifies the differential voltage between a potential command value and a potential signal corresponding to a system voltage detected by a potential detector and outputs the difference as a reactive power compensation control signal, and a reactive power output circuit which controls the voltage of the electric power system by being supplied with the signal output from the voltage control circuit via a phase control circuit so as to compensate for reactive power in the power system.
2. Description of the Prior Art
Conventionally, a reactive power compensator of this kind is constituted by a closed loop voltage control system in order to constantly maintain the voltage of the electric power system. FIG. 4 is a block diagram of a conventional reactive power compensator which is supplied with a potential command value Vp and which has: an adder SUM for calculating and outputting the difference between the potential command value Vp and a potential signal Vf corresponding to the detected voltage of the electric power system; a voltage control circuit AVR supplied with the output from the adder SUM and outputting a reactive power control signal .DELTA.q for controlling the voltage of the electric power system; a phase control circuit APS for phase-controlling and supplying control pulses to a thyristor to control, on the basis the signal .DELTA.q output from the voltage control circuit AVR, the current flowing through a reactor which outputs reactive power; a reactive power output device SVC for making a lagging reactive current flow through the reactor on the basis of the control pulses supplied from the phase control circuit APS so as to compensate for reactive power of the electric power system; and a transducer VDT for detecting the voltage of the electric power system and effecting level-shifting of the same. SYS represents characteristics of the system and represents a change in the voltage of the system in response to a change in the reactive power. In this voltage control system of the reactive power compensator, the gain of the voltage control circuit and the delay time constant and the like are controlled in order that the cut-off frequency of the loop transfer function of the closed loop will become equal to a value which satisfies the response condition of the system, and that the phase margin at that frequency will become sufficient. An example of this kind of apparatus is described in APPLICATION OF STATIC VAR COMPENSATOR FOR AC/DC INTERCONNECTED POWER SYSTEM CIGRE SC 14 SYMPOSIUM 400-02 (Sept., 1987).
As is apparent from FIG. 4, in the reactive power compensator based on the above conventional art, the gain of the loop changes abruptly in response to a change in the characteristics of the electric power system, i.e., the impedance of the system because of the characteristic SYS of the electric power system inserted in series in the voltage control loop, even if the gain, the lag time constant or the like of the voltage control circuit is controlled in order to obtain desired response performance and stability of the reactive power compensator. The response and stability of the reactive power compensator are thereby changed also. In general, the impedance of the system changes with time depending upon the power demand on the load side. Conventionally, to cope with this problem, the gain, the lag time constant and the like of the voltage control circuit are adjusted so that the desired response and stability are obtained when the loop gain or the impedance of the system are maximized. In the control system thus adjusted, the response performance deteriorates as the impedance of the electric power system becomes smaller, that is, the loop gain becomes smaller. It is thus difficult to optimize the effects of the reactive power compensator.